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The lipid bilayer is made of phospholipids. They are arranged in a double layer because one of their ends is attracted to water while the other is repelled by water.
Intradermal injections are somewhat difficult to correctly administer because the skin layers are so thin that it is easy to accidentally punch through to the deeper subcutaneous layer.
The familiar sounds of your heart are made by the heart's valves as they open and close.
Hippocrates noted that blood separates into four differently colored liquids when removed from the body and examined: a pure red liquid mixed with white liquid material with a yellow-colored froth at the top and a black substance that settles underneath; he named these the four humors (for blood, phlegm, yellow bile, and black bile).
There are 60,000 miles of blood vessels in every adult human.